1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copier, a laser printer, an inkjet printer and the like. The present invention specifically relates to an image forming apparatus which forms an image by combining a plurality of images on a single transfer sheet.
2. Description of the Related Art
In recent years it has become important that image forming apparatuses can reproduce full color images without image dislocation (color drift). Specifically, digital methods use a plurality of image forming units corresponding to each color are installed with integrated elements such as photosensitive members, charging devices, and developing devices, and multiple transfers are accomplished for the image formed by each unit onto a single transfer member. Position error of images formed by each image forming unit can be detected and corrected, which is necessary to obtain high precision in image reproducibility. Therefore, a predetermined registration mark or pattern is formed on the transfer member by each image forming unit, and these marks or patterns are read by detection devices to accomplish positional alignments.
For example, Japanese Laid-Open Patent Application No. 1-141746 discloses a method wherein registration marks extending in a main scan direction and a subscan direction are detected by an inclined charge-coupled device (CCD). Japanese Laid-Open Patent Application No. 4-131750 discloses a method wherein registration marks comprising straight lines extending in a main scan direction and diagonal lines inclined relative to said straight lines and extending in the same direction are formed. These registration marks are detected by a single photosensor, and the amount of dislocation is calculated and corrected.
In the former method, because the registration marks are read by a charge storage type CCD, the sampling period is long and detection precision does not allow fine resolution, or the sampling data rate must increase to accomplish fine resolution, thereby enlarging the processing unit and greatly increasing cost. The latter method, on the other hand, uses a non-integrating sensor, and avoids the aforesaid disadvantages in conjunction with the sampling period, but because the registration mark is detected at one point in the main scan direction, there are reading position errors in the main scan direction when the transfer member travels at irregular speed in the subscan direction. Furthermore, there is the possibility of inadequate detection when the pattern, skips so as to leave out part of the pattern because a combination of horizontal and vertical lines, or diagonal lines are used as the registration pattern.